Method for Offset Timing of Simultaneous Seismic Source Firing

ABSTRACT

A technique enables measuring of the actual firing sequence times in a seismic survey application. The actual firing sequence times are then employed in simultaneous source resolution methods. For example, simultaneous seismic sources may be deployed in a survey region. The seismic sources are then fired, and the actual firing times are determined and recorded for use in optimizing the seismic survey.

BACKGROUND

In a variety of environments, seismic surveys are performed to gain abetter understanding of subterranean geological formations. In marineenvironments, for example, seismic surveys are conducted to improve theunderstanding of geological formations located beneath a body of water.In seismic survey applications, seismic sources are employed to createpulses of energy, and data on the energy from the seismic sources isrecorded. The recorded information is used to improve the quality of theseismic survey by, for example, optimizing signal resolution.

When employing simultaneous source methods of seismic acquisition,various techniques are employed to establish source firing offsets,often called source sequences. These offsets in time may then be used tofind, resolve, and/or separate any of the simultaneous sources fromother coherent and incoherent noise sources in the seismic records. Theoffsets may either be systematic or pseudo-random. Both approaches areactually deterministic in that the methodology is designed with respectto the systematic approach and constrained with respect to thepseudo-random approach. The approaches are employed to optimize methodsof signal resolution; however errors or inaccuracies may result from thesystematic and/or pseudo-random approaches.

SUMMARY

In general, the present invention provides a methodology for measuringthe actual firing sequence times which are then employed in simultaneoussource resolution methods. For example, simultaneous seismic sources maybe deployed in a survey region. The seismic sources are then fired, andthe actual firing times are determined and recorded for use in asimultaneous source resolution regime designed to enable optimization ofa seismic survey operation.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain embodiments of the invention will hereafter be described withreference to the accompanying drawings, wherein like reference numeralsdenote like elements, and:

FIG. 1 is a schematic view of a seismic vessel pulling a plurality ofseismic sources in a marine survey area, according to an embodiment ofthe present invention;

FIG. 2 is a schematic illustration of a system for determining theactual firing sequence times of seismic sources, according to anembodiment of the present invention;

FIG. 3 is a schematic illustration of an embodiment of certain types ofcomponents employed to determine actual firing times, according to anembodiment of the present invention;

FIG. 4 is schematic illustration showing data flow of timing pulseinformation, according to an embodiment of the present invention;

FIG. 5 is a flowchart illustrating one example of a methodology used toimprove a seismic survey operation, according to an embodiment of thepresent invention; and

FIG. 6 is a flowchart illustrating another example of a methodology usedto improve a seismic survey operation, according to an embodiment of thepresent invention.

DETAILED DESCRIPTION

In the following description, numerous details are set forth to providean understanding of the present invention. However, it will beunderstood by those of ordinary skill in the art that the presentinvention may be practiced without these details and that numerousvariations or modifications from the described embodiments may bepossible.

The present invention generally relates to a technique for improving thequality of seismic surveys. A plurality of seismic sources is used forgenerating seismic signals which propagate into the earth. The seismicsignals may be generated in both land and marine environments forreflection by subsurface seismic reflectors. The reflected signals arerecorded by seismic detectors which may be disposed along a surface ofthe sea in marine applications or along a land surface in landapplications. The recorded data is processed to obtain usefulinformation related to subsurface characteristics that can aid in thesearch for oil and gas deposits.

To improve the methods for finding any particular seismic source in aseismic record, the present methodology determines the actual firingtimes of the seismic sources instead of using the planned systematic orpseudo-random firing times. According to one embodiment of the presentinvention, the actual firing times are determined and subsequentlyemployed in simultaneous source resolution methods. By using the actualfiring times, the source firing offsets in time can be used to moreaccurately find, resolve, and/or separate any one of the simultaneoussources from the other coherent and incoherent noise sources in theseismic records. In other words, the source signal separation methodsare improved by using the actual seismic source firing time offsetsrather than the planned offsets.

Determining the actual seismic source firing offsets, as describedbelow, may be achieved with a timing algorithm that measures gun firingclosure using a precision clock combined with an accounting for systemtransmission delays. Time differences between planned source firingevents and the actual realization of those events are recorded alongwith the timing residuals. The recorded data may then be made availableto a variety of simultaneous source separation methods to improve thesignal-to-noise ratio of resolved individual source signals.

Referring generally to FIG. 1, a general seismic survey system 20 isillustrated to show one example of a method for acquiring seismic data.In this example, seismic system 20 is a marine system, although landbased systems also may be utilized to achieve the desired information onsubterranean formations. As illustrated, seismic system 20 comprises atow vessel 22 for towing a plurality of seismic sources 24 which areused as simultaneous sources for generating seismic signals. In themarine application illustrated, sources 24 may comprise guns that can befired to initiate waves of energy. In some applications, vessel 22 alsomay be used to tow one or more streamers 26 having a plurality ofsensors 28 that are used to detect reflected seismic source signals. Inother applications, the streamers 26 may be towed with a separatevessel.

Seismic system 20 also comprises a seismic data acquisition system 30,as illustrated schematically in FIG. 2. Data acquisition system 30 isused to determine and record actual firing times along with otherseismic survey data. According to one embodiment, data acquisitionsystem 30 comprises a control system 32 that may be a computer-basedsystem. The control system 32 may be used to automatically calculateactual seismic source firing times, to record those times in the seismicrecords, and/or to utilize the actual firing sequence times in varioussimultaneous source resolution methods/algorithms. By way of example,control system 32 is utilized in performing simultaneous sourceseparation methods to improve the signal-to-noise ratio of resolvedindividual source signals. In many applications, the actual firing timesmay be employed to optimize the separation of simultaneously recordedsources.

By way of general example, control system 32 comprises a centralprocessing unit (CPU) 34 which is operatively connected to a variety ofcomponents used in initiating firing and in detecting, relaying andrecording data related to the seismic source firing. CPU 64 also may becoupled with a control system memory 36, an input device 38, and anoutput device 40. Input device 38 may comprise a variety of devices,such as a keyboard, mouse, voice-recognition unit, touchscreen, otherinput devices, or combinations of devices. Output device 40 may comprisea visual and/or audio output device, such as a monitor having agraphical user interface. The output device 40 may be used to display toan operator information related to actual firing times and to otherparameters related to a given seismic operation. The processing may beaccomplished on a single device or multiple devices at the surveyregion, away from the survey region, or with some devices located at thesurvey region while others are located remotely. Furthermore, controlsystem 32 may be incorporated partially or wholly into the various othercomponents utilized in data acquisition system 30.

As illustrated, other components of seismic data acquisition system 30comprise a plurality of modules designed to handle various aspects ofthe seismic source signal initiation, detection and recording. Forexample, data acquisition system 30 may comprise a source control module42 which is designed to provide source firing control and digitizationof sensor data for source, e.g. gun, firing times, source depth, andnear field signature. As illustrated, each source control module 42comprises electronics 44 specifically used to provide the source firingcontrol as well as the digitization of sensor data.

Another module of the seismic data acquisition system 30 is the datarecording module 46. The data recording module 46 retrieves and storesseismic data as measured by the sensors 28 positioned along streamers26. The data recording module 46 also stores data from the in-seadigital source system, e.g. data obtained from electronics 44 on sourcefiring time, source depth, near field signature, and other desired data.It should be noted that data recording module 46 may be combined with orused in cooperation with memory 36 of control system 32. In theembodiment illustrated, seismic data acquisition system 30 furthercomprises a navigation module 48. Navigation module 48 may be used for avariety of services, including navigation, source positioning, databinning, and related quality assessment services.

In the example illustrated, each of the main data acquisition systemmodules 42, 46, 48 has a time control unit 50. The time control units 50are connected to a timing relay and pulse box 52 which collects timingdata from the data acquisition system modules 42, 46, 48. The timingrelay and pulse box 52 comprises timing counters 54 that are stoppedupon receiving timing pulse events from the various data acquisitionsystem modules. The specific timing of pulse events received from themodules 42, 46 and/or 48 can be compared relative to each other andrelative to planned shot times, i.e. planned seismic source firingtimes. The control system 32 may be combined with timing relay and pulsebox 52 or maintained separately. Additionally, the control system 32 maybe utilized to perform a variety of statistical analyses based on theplanned and measured data related to firing times. Depending on thespecific data acquired, the algorithms/statistical analyses may vary,but the information gathered is readily utilized in determining theactual firing times of sources 24 according to conventional statisticalanalysis techniques.

In the present example, the timing relay and pulse box 52 also isoperatively coupled with an integrated timing receiver 56, asillustrated in FIG. 3. The integrated timing receiver 56 runs a highfrequency oscillator 58 and is connected to a global positioning system60. In seismic survey applications described herein, global positioningsystem 60 may be used to obtain accurate positions and times, which datais then used by, for example, control system 32. Accordingly, the globalpositioning system 60 is employed to obtain desired data on the timingpulses or other signals related to the firing of simultaneous sources 24and/or to the detection of reflected seismic energy at sensors 28.

With the integrated timing receiver 56, the high frequency oscillator 58enables maintenance of accurate timing within microseconds, e.g. lessthan 30 microseconds, of the global positioning system constellationtime. Additionally, the time control units 50 of seismic dataacquisition system 30 establish individual clocks for individual dataacquisition modules, e.g. modules 42, 46, 48. In this example, theindividual clocks are all kept within a few microseconds, e.g. less than10 microseconds, of the integrated timing receiver time.

Referring generally to FIG. 4, an example is provided to illustrate thedata flow of hardware timing pulse information. The accurate collectionof the various timing data, e.g. timing pulses, combined with therecorded firing times can be used in a statistical analysis to determineactual firing times. For example, control system 32 may be programmedwith available statistical methods to provide a statistical analysis ofthe data in real-time to accurately determine actual firing times ofsources 24.

In the example illustrated in FIG. 4, the navigation module 48 controlsand distributes the nominal source firing times to the other subsystemsof the seismic data acquisition system 30. For example, the nominalsource firing times are distributed to source control module 42 and datarecording module 46. However, the nominal source firing times also maybe delivered to the timing relay and pulse box 52 and/or to otherpotential data acquisition modules, as represented by block 62. In thisexample, the navigation module 48 also causes initiation of a gatingtimer 64 of the timing relay and pulse box 52 to catch source timepulses from any of the data acquisition system modules that are equippedwith a time control unit 50. In one embodiment, the navigation module 48causes the timing relay and pulse box 52 to initiate gating timer 64with a master pulse before the planned nominal shot/firing time asrepresented by block 66.

The firing of seismic sources 24 is controlled by source control module42. However, after the shot has been fired under control of the sourcecontrol module 42, the source control module electronics 44 are used todetect the emitted pressure wave. Upon detection of the emitted pressurewave, the electronics 44 (in cooperation with the control unit 50 ofsource control module 42) send a pulse back to the timing relay andpulse box 52, as represented by block 68. Upon receipt by the timingrelay and pulse box 52, the corresponding timing counter 54 is stopped,as represented by block 70.

Additionally, the recording module 46 measures the peak pressure timeand sends its own timing pulse back to the timing relay and pulse box 52via its time control unit 50, as represented by block 72. When thetiming pulse from recording module 46 is received by timing relay andpulse box 52, the corresponding timing counter 54 is stopped, asrepresented by block 74.

If other data acquisition system modules 62 are employed, those modulesalso send an appropriate timing pulse back to timing relay and pulse box52 via their timing control units 50, as represented by block 76. Again,receipt of the pulse/signal causes the corresponding timing counter 54to immediately stop, as represented by block 78. Consequently, timingrelay and pulse box 52 collects a variety of data on both the planned ornominal firing times of sources 24 and on observed events from aplurality of data acquisition system modules, e.g. modules 42, 46, 48,62. The data collected may be processed on a suitable processor system,such as control system 32, via a suitable algorithm or statisticalanalysis program selected according to the specific timing dataobtained, as represented by block 80. A variety of available, precisionstatistical evaluation methods may be employed to evaluate thedifferences between pulse events relative to the planned shot time andrelative to each other to determine the actual firing times of sources24. Regardless of the specific data acquisition system or technique, theactual firing sequence times are determined and employed in the selectedsimultaneous source resolution regime/method to optimize the seismicsurvey operation.

In the example illustrated in FIG. 4, the relative times of the pulsesfrom the data acquisition system modules and the recorded firing timesmay be evaluated in real-time according to the selected precisionstatistical method. Depending on the adequacy of the clocks in timecontrol units 50, timing relay and pulse box 52, and/or globalpositioning system 60, the actual firing times may be determined withgreat accuracy. For example, actual firing times may be determined withan accuracy of plus/minus 100 microseconds. The actual firing times arethen stored together with the seismic records in, for example, memory36. The recorded actual firing times may be retrieved and used toenhance the seismic survey by, for example, optimizing the separation ofsimultaneously recorded sources.

One example of a general approach to employing the methodology describedherein is illustrated in the flowchart of FIG. 5. In this example,seismic sources 24 are initially employed in a desired seismic surveyregion, such as a marine survey region, as represented by block 82. Thesimultaneous seismic sources are shot, e.g. fired, to input a seismicenergy into the seismic survey region, as represented by block 84. Theactual firing times of the seismic sources are then determined tofacilitate an improved seismic survey through, for example, optimizationof the separation of simultaneously recorded seismic sources, asrepresented by block 86.

The specific approach used in obtaining and utilizing the actual firingtimes may vary from one seismic survey application to another. Themethodology also may be adjusted to accommodate land based or marinebased seismic surveys and to accommodate attributes of the specificenvironment. In a typical marine environment, for example, simultaneoussources 24 are employed in a marine survey region, as represented byblock 88 of the flowchart illustrated in FIG. 6.

Planned source firing offsets are then established for the seismicsurvey application, as represented by block 90. The simultaneous sources24 are then fired, as represented by block 92. Subsequently, the actualfiring times of the simultaneous sources 24 are determined, asrepresented by block 94. The actual firing times may be determinedaccording to a selected procedure, such as the procedure described abovewith reference to FIG. 4. The actual firing times, along with additionalsurvey data, is then stored in a seismic record, as represented by block96. The stored data on actual firing times is used to improve the surveyresults. By way of example, the actual firing time data enablessubstantial improvement in simultaneous source resolution methods, asrepresented by block 98.

The embodiments discussed above provide examples of systems, componentsand methodologies that may be used to improve the results of seismicsurveys. Depending on the specific application and environment, thearrangement of systems and components may be changed or adjusted toaccommodate the characteristics of the application and environment. Forexample, the number of simultaneous sources 24 may be two or more asdesired for a specific application. Additionally, guns or other devicesmay be used to impart seismic energy into the surrounding environment.Similarly, a variety of sensors and arrangements of sensors 28 may beselected to detect the reflected energy waves.

Additionally, seismic data acquisition system 30 may comprise a varietyof control systems, data acquisition modules, timers and othercomponents to facilitate detection, measurement and storage of datarelated to simultaneous source firing. Furthermore, data may be relayedto the timing relay and pulse box (or other suitable component) via avariety of pulse/signal communication techniques. The overall dataacquisition system also may be designed in a variety of ways to utilizethe global positioning system for transfer of data in real time.

Although only a few embodiments of the present invention have beendescribed in detail above, those of ordinary skill in the art willreadily appreciate that many modifications are possible withoutmaterially departing from the teachings of this invention. Accordingly,such modifications are intended to be included within the scope of thisinvention as defined in the claims.

1. A method of performing a seismic survey operation, comprising:employing simultaneous sources in a survey region; establishing sourcefiring offsets; firing the simultaneous sources according to the sourcefiring offsets; determining the actual firing sequence times of thesimultaneous sources; storing the actual firing sequence times tosupplement a seismic record; and employing the actual firing sequencetimes in a simultaneous source resolution regime to optimize a seismicsurvey operation.
 2. The method as recited in claim 1, wherein employingcomprises using the actual firing sequence times to optimize theseparation of simultaneously recorded sources.
 3. The method as recitedin claim 1, wherein firing comprises utilizing a source control modulehaving distributed in-sea electronics located at each source.
 4. Themethod as recited in claim 3, wherein storing comprises storing theactual firing times on a data recording module.
 5. The method as recitedin claim 4, further comprising utilizing a navigation module to controland distribute a nominal source firing time to the source control moduleand the data recording module.
 6. The method as recited in claim 5,further comprising using a time control unit with each of the sourcecontrol module, data recording module, and navigation module toestablish individual clocks.
 7. The method as recited in claim 6,further comprising placing the time control units in communication witha timing relay and pulse box having timing counters which are stopped toestablish pulse events relative to each other and to planned firingtimes of the simultaneous sources.
 8. The method as recited in claim 7,further comprising utilizing an integrated timing receiver coupledbetween the timing relay and pulse box and a global positioning system.9. A method, comprising: using a source control module to control shotsat a plurality of seismic sources in a seismic source array; detectingpressure waves emitted from the plurality of sources; sending timingpulses corresponding to the pressure waves to a timing relay and pulsebox; employing a data recording module to measure peak pressure timesand to send corresponding peak pressure time pulses to the timing relayand pulse box; and stopping timing counters in the timing relay andpulse box to determine the timing pulse events for calculating theactual firing times of the plurality of sources.
 10. The method asrecited in claim 9, further comprising comparing timing pulse events toeach other and to planned firing times of the plurality of seismicsources.
 11. The method as recited in claim 10, further comprisingoutputting data on the actual firing times to a display for use by anoperator.
 12. The method as recited in claim 10, further comprisingobtaining data related to timing pulse events via a global positioningsystem and an integrated timing receiver coupled to the timing relay andpulse box.
 13. A method of performing a seismic survey operation,comprising: employing seismic sources in a survey region; determiningactual firing sequence times; and employing the actual firing sequencetimes in a simultaneous source resolution regime to optimize a seismicsurvey operation.
 14. The method as recited in claim 13, furthercomprising recording the actual firing sequence times in a seismicrecord.
 15. The method as recited in claim 14, wherein employingcomprises recording the actual firing sequence times to improvesimultaneous source resolution.
 16. The method as recited in claim 13,further comprising firing the seismic sources at planned source firingoffsets.
 17. The method as recited in claim 13, wherein determiningcomprises using a plurality of time control units to provide timingpulse data to a timing relay and pulse box.
 18. The method as recited inclaim 13, wherein employing comprises using the actual firing sequencetimes to optimize source firing time offsets.